Electro-magnetic shunt for telephone-receivers



(No Model.)

T. D. LOGKWOODL ELEGTRO MAGNETIO SHUNT EOE TELEPHONE RBGEIVERS.

No. 393,165. Patented Nov. 20, 1888..

Winwses. Inwenfior N. PITERS. Phnwliflmgnphur. Washlngton, Dv C.

NITE'D! STATES ELECTRO-MAGNETIC SHUNT FOR TELEPHONE-RECEIVERS.

SPECIFICATION forming part of Letters Patent No.393,l65, dated November 20, 1888.

Application filed August 10, 1888. Serial No. 282,407. (No model.) A

To to whom it may concern- Be it known thatI, THOMAS D. LOOKWOOD, of Melrose, in the county of Middlesex and State of Massachusetts, have invented certain Improvements in Electro-Magnetic Shunts for Telephone-Receivers, of which the following is a specification.

This invention relates to means for relieving telephone-instr uments' from the pernicious and disturbing effects of induction from other circuits. It is especially serviceable in cases where the said disturbing effects accrue from circuits carrying electric-light and power currents.

The invention is based upon the well-known fact in applied electricity that telephonic electric currents, by reason of their rapid rate of change per unit of time, pass through electromagnetic helices with great difliculty and with considerable loss of volume and power. This phenomenon is of course due to the counter magneto-electric currents which are set up in the electro-magnet coil, and which flow to the main circuit at every change in the telephonic current. Since these changes occur at the rate of about twenty-eight thousand per second of time, and since a counter-current is developed at each change, it follows that an ordinary electro-magnet is in no inconsiderable degree opaque to telephonic or voice currents. It is also based upon the electrical axiom that when two routes are open to a current of electricity under ordinary conditions the said current will divide between them in the inverse ratio of their resistances or directly as their respective conductivities. Heretofore attempts have been made to reduce the disturbances due to induction by shunting the receiving-telephone by a derived circuit of suitable resistance. It has, however, invariably been found that while the induction was indeed diminished the legitimate or voice current was also diminished and in a like proportion, and thus the gain in comparative freedom from disturbance was in a great measure counterbalanced by the loss in the volume or strength of the reproduced conversation. By the employment of myinvention I provide that the disturbing current shall be practically dissipated or led off by a by-path, while the voice-currents are caused to pass through the telephone helix without sufl'ering material diminution. This I accomplish by shunting the receivingtelephone with a properly-pro portioned electro-magnet, by providing that this shunt shall preferably be formed of an earth branch; that it shall be derived from the main line at a point between the secondary helix of the induction-coil and the helix of the receiving-telephone, the induction-coil being located in the circuit with the most advantage outside of or external to the receiving-telephone, and by introducing resistance into the telephone branch of sub-stations so provided.

Figures 1 and 2 are diagrams indicatingsuccessive steps in the history of theinvention.

Fig. 3 is a diagram of a line fitted with theinduction-suppressor at three stations. Figs. 4. and 5am respectively a cross-sectional elevation and a perspective View of the peculiar form of the electromagnetic shunt which I employ or may employ, and which, in combination with a telephonic receiver, constitutes a special feature of my invention; and Fig. 6 is a cross-section of a modification thereof.

In Figs. 1, 2, and 3,10 representsatelephoneline, while IV in Fig. 1 represents a wire car rying electric-light or power currents, exercising induction upon the telephone and producing a loud musical sound in the receivingtelephoues, whereby the reproduction of speech is greatly interfered with. This wireW is not shown in the other figures, although its presence is to be assumed. The receiving-telephone t andthe transmittingtelephone t are in the telephone-circuit which terminates at the earth G. From a point, 4, on the line to a branch, derived, or shunt circuit, a, is led to earth at G, passing through an electro-magnet so constructed as to have a very high coefficient of self-induction, while it may be of comparatively low resistance relatively to the resistance of the telephone branch or extension T. An ordinary or steady electric current flowing in the circuit to would of course divide at the point 4 inversely in the ratio of the resistances of the two branches T and a, regardless of other conditions; but with a telephonic current which is constantly and with great frequency changing in character the case is different. The introduction of the electro-magnet 1) into the derived circuita makes the said derived circuit practically impervious to the telephonic current of the main line, for reasons hereinbefore stated. Consequently I am enabled to construct the electro-magnet b with comparatively coarse wire,offering but a very low resistance relatively to the total resistance of the telephone-extension T. Notwithstanding this low resistance, however, very little of the legitimate telephonic current will leak away by this route to earth by reason of the imperviousness to such currents which I have described, this being due to the self-induction of the electro-magnct, or, in other words, to the counter-currents generated in the coils by the continuously-magnetic variation of the core effected by the passing voicccurrents.

While the disturbing induced currents are not absolutely constant and uniform, and do not, therefore, pass the elcctro-magnet with as great a facility as if they were constant,

. still their frequency of change is greatly beturbing current which leaks away over the low that of the telephonic currents, as may be demonstrated from the pitch of the musical sound which they produce in the telephone. Asa consequence, they are toa much greater extent amenable to the general law of derived circuits which I have hereinbefore stated, and the portion of the entire disderived circuit is a much greater fraction of the whole than can be the case with the telephonic current. Thus I am enabled to subdue to a great extent the undesirable noises of the telephone without any appreciable diminution of volume in the reproduced speech; and I have found in practice that in each individual case the resistance and self-induction of the shunting'magnet I) can without diiiiculty be so proportioned that the disturbance can be subdued to such a degree as to be scarcely perceptible,while the telephonic current is apparently undiminished in power. Moreover, the clectro-motive force of the induced or disturbing current is not so high as is that of the telephonic current; hence it loses much more in strength by the intcrposition of resistance.

The arrangement shown in Fig. 1 has been found an effective suppressor of induction; but it is evident that it materially interferes with the efficiency of the transmitter when the latter is placed as shown, for, while the derived circuit 6t does not materially depreciate the power of the receiving-telephone as regards voice-currents, it seriously cripples the power of the transmitter; and this was to be expected, because the leak through the magnet to earth intercepts the current generated by the transmitter, it being a much shorter circuit from the junction-point through the derived circuit than over the line, while the incoming current, which is required to act on the receiver, does not strike the divergent route throughthe shunt-circuit until the line con stitutingthe greater portion of the circuit is passed over, so that, even though the said shunt-circuit were a simple resistance, very little, comparatively,of the said current would be lost. But it is now clear that it is not necessary to shunt both transmitter and receiver. It is the latter appliance only which requires protection from the induced currents, and I find it then convenient and preferable to connect the appliances, as in Fig. 2, in which the transmitter-circuits are drawn in detail. The transmitter connects with the main line through the secondary helix 0 of an inductioircoil, 1" being the primary helix thereof, (in circuit with the transmitter proper, t, and the local battery (1,) r being the iron core. The derived circuit is in this figure and in practice connected with the line at a point between the secondary coil 0 and the receivingtelephone 1%, the transmitter-coil being in theoutward position-that is, included in the circuit in a position nearer than the receiver to the external portion of the line. So connected, the result is that the noise is hushed down, as in the former case, the major part of the disturbing current leaking away through the shunt. The telephone-current is not materially lessened, any slight depreciation in volume beingfully made up by increased sharpness and by the quietness of the line, and, the secondary transmitter coil being new uncontrolled by the shunt, the power of the transmitter is now actually greater than when con nccted as usual.

I account for the fact that the power of the transmitter is actually increased under the above conditions in the following manner: In the first place, the two branches in the rear of the transmitter-coil furnish two routes to earth, and thus effectually guarantee that the resistance of the earth'terminal of the line shall be kept down; and in the second place a telephone-transmitter coil so located has a greater tendency to propagate the impulses it develops in a forward direction, because the two routes in its rear both include an electromagnetic coil, andtherefore both act asa dam upon the currentin the direction of the ground end. It is therefore a decided advantage in several ways to branch the shunt-wire from a point between the transmitter and receiver coils and 'to provide that the former shall be outermost.

In Fig. 3 is represented a line having three stations, two of which, 1 and 3, are terminal, while one, 2, is way. It is obviously impracticable to establish an earth-shunt including a suitable electro -n1agnet at way stations. Therefore, although, for reasons hereinbefore stated, an earth branch or shun-t is to be preferred, I employ at way-stations a loop-shunt round the telephone-helix only. The said shunt extends from a point, a, through the electromagnetic shunt b to a point, 4", on the other side of the telephone t. In order that during the day-time, when electric-light induction is not ordinarily prevalent, the shunt may, if desired, be thrown oft for the purpose of obtaining the full value of the telephone, I provide a switch, 8, in the shunt-circuit.

In some cases, where the electric-lighting current is generated by dynamos rotating at a very high rate of speed, it is found that the rate of change per unit of time'of the induced current is also much higher. This of course brings the said rate nearer to that of the telephonic current. When this occurs, it becomes necessary to provide an electro magnetic shunt, b, the resistance whereof is materially lower than would otherwise be required. When the said shunt is so low that a further reduction would too greatly weaken the telephonccurrent, availing myself of the wellknown fact that the said telephone-current has a very high electro-motive force, and is thus practically unaifected by the introduction of simple resistance-coils without iron cores, I introduce resistance-coils f, wound differentially to avoid self-induction, into the telephone branch; and Ifind that in practice this achieves the desired end and diminishes the induction-currents in a much greater degree than it does the legitimate or voice currents.

In Figs. and 5 is shown one form of electro-magnet well adapted for inclusion in the derived circuit which I have described. It is composed of two spools, h, of insulated wire 2', the helices of the two being connected in series. Each has a soft-iron core, j, and the said cores are united at both ends by iron plates 9 and It, thus (if we may so speak) closing the magnetic circuit. Each spool also has an inclosing-case, x, of iron. I have attained good results by the useof this coil. Instead, however, of uniting the cores of the two coils by iron cross-bars or yokes, I may provide simply separate iron disks, m, at each end of the two coils, as in Fig. 6. hen this is done, a convenient way is to form one of the said disks integral with the inclosing iron sheath a: and to fasten the other to the core, which may then be slid in and out of the coil to regulate the inductive power of the appliance in a manner well understood. When the form of coil shown in Fig. 6 is adopted, one or more may be used, connected in series, as any special case may require.

In practice with the average telephone-line I have found the best results, both with respect to the reduction of disturbance and the maintenance of the efliciency of the talking, to

accrue from a small and short coil measuring about ten ohms resistance, provided with an iron external sheath, iron disks inclosing the ends, and a solid iron core, the said coil being about one and a half inch long and one and one-eighth inch thick externally. This coil was wound with No. 26 Brown & Sharpe silkcovered wire.

I am aware that patent of S. 1?. Thompson,

pose of increasing the sensitiveness of such a telephone, said shunt including a resistance 7c of such nature as to oppose the passage of the voice-currents, while offering practically no opposition to the steady battery-currents. The arrangement shown in Fig.1 of the present application is, however, regarded as a novel expedient for suppressing induced currents when the telephone-circuit is in proximity to a disturbing wire.

Having thus described my invention, I c1aim 1. The combination, in a telephone-circuit,v of a transmitting-instrument operating upon the main line by means of an induction-coil having its secondary helix included in the circuit of said main line, a receiving-telephone 8 also in said circuit, and a derived circuit to earth, including an electro-magnet branching from said main line at a point between the secondary helix and the receiving-telephone helix, substantially as described.

2. In a telephone-circuit, the combination, substantially as hereinbefore described, of a telephone-transmitter, a telephone-receiver, and a derived circuit to earth, including a suitably constructed and proportioned electromagnet, the said transmitter being connected with the main circuit at a point external to or nearer the line than the receiver, and the said derived circuit being branched from the main circuit at a point between the transmitter and 10 receiver.

3. In a telephone-circuit, and in combination with the line and the transmitter and receiver in said line, a derived circuit branched from the line between the receiver and trans- 1o 5 mitter and shunting the receiver, and an electro-magnet in said derived or shunt circuit, substantially'as described.

4. In a grounded telephone-circuit, and in combination with the main line, the transmit- 1 1o ter in said line, and the receiver also in said line between the transmitter and the ground, a derived circuit branched from the line between the receiver and transmitter and shunting the receiver, substantially as described.

5. A telephone-circuit and a transmitter and receiver included therein at the several stations thereof, an electromagnetic shunt, substantially as specified, around the receiver only and not around the transmitter, and a 12 switch in circuit with the said shunt for the purpose of disconnecting the same when re quired.

6. The combination, at a telephone-station, substantially as hereinbefore described, of a 125 main circuit, a shunt or derived circuit, a receiving-telephone common to and included in both circuits, and an electro-magnetic appliance comprising one or more spools of insulated wire surrounding iron cores and inclosed I 0 than that of the telephone-helix.

in iron envelopes in the said shunt-circuit, In testimony whereofIhavesigned my name 10 the said spools being of lower total resistance to this specification, in the presence of two subscribing witnesses, this 28th day of July, 1888.

7. The combination of a main t-elephone-circuit, a receiving-te1ephone,and a resistance or rheostat included therein, and a derived cir- Witnesses:

cuit shunting the said telephone and resist- I GEO. WILLIs PIERCE, ance, substantial] y as and for the purposes V. M. BERTHOLDE. specified.

THOSQ. D. LOOKWOOD. 

